Priority-based non-adjacent merge design

ABSTRACT

Devices, systems and methods for constructing low-complexity non-adjacent merge candidates are described. In a representative aspect, a method for video processing includes receiving a current block of video data, selecting, based on a rule, a first non-adjacent block that is not adjacent to the current block, constructing a first merge candidate including motion information based on the first non-adjacent block, identifying a second non-adjacent block that is not adjacent to the current block and different from the first non-adjacent block, based on determining that the second non-adjacent block fails to satisfy the rule, refraining adding a second merge candidate derived from the second non-adjacent block, constructing a merge candidate list based on the first non-adjacent block, and decoding the current block based on the merge candidate list.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/IB2019/055573 filed on Jul. 1, 2019 which claims the priority to andbenefits of International Patent Application No. PCT/CN2018/093944,filed on Jul. 1, 2018. All the aforementioned patent applications arehereby incorporated by reference in their entireties.

TECHNICAL FIELD

This patent document is directed generally to image and video codingtechnologies.

BACKGROUND

Digital video accounts for the largest bandwidth use on the internet andother digital communication networks. As the number of connected userdevices capable of receiving and displaying video increases, it isexpected that the bandwidth demand for digital video usage will continueto grow.

SUMMARY

Devices, systems and methods related to identifying non-adjacent mergecandidates to decrease memory and complexity requirements and increasecoding efficiency are described. For example, the presently disclosedtechnology discloses rules to select non-adjacent merge candidates tokeep the size of the line buffer under a threshold. The describedmethods may be applied to both the existing video coding standards(e.g., High Efficiency Video Coding (HEVC)) and future video codingstandards or video codecs.

In one representative aspect, the disclosed technology may be used toprovide a method for video processing. This method includes receiving acurrent block of video data; selecting, based on a rule, a firstnon-adjacent block that is not adjacent to the current block;constructing a first merge candidate comprising motion information basedon the first non-adjacent block; identifying a second non-adjacent blockthat is not adjacent to the current block and different from the firstnon-adjacent block; based on determining that the second non-adjacentblock fails to satisfy the rule, refraining adding a second mergecandidate derived from the second non-adjacent block; constructing amerge candidate list based on the first non-adjacent block; and decodingthe current block based on the merge candidate list.

In another representative aspect, the disclosed technology may be usedto provide another method for video processing. This method includesreceiving a current block of video data; selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block;constructing a first merge candidate comprising motion information basedon the first non-adjacent block; selecting a second non-adjacent block,wherein the first non-adjacent block and the second non-adjacent blockare not adjacent to the current block, wherein the first non-adjacentblock is coded with a first mode, and the second non-adjacent block iscoded with a second mode, and wherein the first non-adjacent block isselected prior to selecting the second non-adjacent block; andconstructing a second merge candidate comprising motion informationbased on the second non-adjacent block.

In another representative aspect, the disclosed technology may be usedto provide another method for video processing. This method includesreceiving a current block of video data; selecting, based on rules, afirst subset of non-adjacent blocks and a second subset of non-adjacentblocks that are not adjacent to the current block; and constructing afirst subset of merge candidates comprising motion information based onthe first subset of non-adjacent blocks and a second subset of mergecandidates comprising motion information based on the second subset ofnon-adjacent blocks, wherein the first subset of merge candidates isadded into a merge candidate list after a merge candidate of a firsttype, and wherein the second subset of merge candidates is added intothe merge candidate list after a merge candidate of a second type.

In another representative aspect, the disclosed technology may be usedto provide another method for video processing. This method includesreceiving a current block of video data; selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block;constructing at least one motion vector predictor candidate based on thefirst non-adjacent block, wherein the at least one motion vectorpredictor candidate is used as a predictor in an advanced motion vectorprediction (AMVP) mode; and decoding a block associated with the AMVPmode based on the at least one motion vector predictor candidate.

In yet another representative aspect, the above-described method isembodied in the form of processor-executable code and stored in acomputer-readable program medium.

In yet another representative aspect, a device that is configured oroperable to perform the above-described method is disclosed. The devicemay include a processor that is programmed to implement this method.Other features that are preferably implemented by various embodimentsare described in the present document.

The above and other aspects and features of the disclosed technology aredescribed in greater detail in the drawings, the description and theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example block diagram of a typical High Efficiency VideoCoding (HEVC) video encoder and decoder.

FIG. 2 shows examples of macroblock (MB) partitions in H.264/AVC.

FIG. 3 shows examples of splitting coding blocks (CBs) into predictionblocks (PBs).

FIGS. 4A and 4B show an example of the subdivision of a coding treeblock (CTB) into CBs and transform blocks (TBs), and the correspondingquadtree, respectively.

FIGS. 5A and 5B show an example of the subdivisions and a correspondingQTBT (quadtree plus binary tree) for a largest coding unit (LCU).

FIGS. 6A-6E show examples of partitioning a coding block.

FIG. 7 shows an example subdivision of a CB based on a QTBT.

FIGS. 8A-8I show examples of the partitions of a CB supported themulti-tree type (MTT), which is a generalization of the QTBT.

FIG. 9 shows an example of constructing a merge candidate list.

FIG. 10 shows an example of positions of spatial candidates.

FIG. 11 shows an example of candidate pairs subject to a redundancycheck of spatial merge candidates.

FIGS. 12A and 12B show examples of the position of a second predictionunit (PU) based on the size and shape of the current block.

FIG. 13 shows an example of motion vector scaling for temporal mergecandidates.

FIG. 14 shows an example of candidate positions for temporal mergecandidates.

FIG. 15 shows an example of generating a combined bi-predictive mergecandidate.

FIG. 16 shows an example of constructing motion vector predictioncandidates.

FIG. 17 shows an example of motion vector scaling for spatial motionvector candidates.

FIG. 18 shows an example of motion prediction using the alternativetemporal motion vector prediction (ATMVP) algorithm for a coding unit(CU).

FIG. 19 shows an example of the identification of a source block andsource picture

FIG. 20 shows an example of a coding unit (CU) with sub-blocks andneighboring blocks used by the spatial-temporal motion vector prediction(STMVP) algorithm.

FIG. 21 shows an example of bilateral matching in pattern matched motionvector derivation (PMMVD) mode, which is a special merge mode based onthe frame-rate up conversion (FRUC) algorithm.

FIG. 22 shows an example of template matching in the FRUC algorithm.

FIG. 23 shows an example of unilateral motion estimation in the FRUCalgorithm.

FIG. 24 shows an example of the decoder-side motion vector refinement(DMVR) algorithm based on bilateral template matching.

FIG. 25 shows an example of spatial neighboring blocks used to derivethe spatial merge candidates.

FIG. 26 shows exemplary pseudocode for adding non-adjacent mergecandidates.

FIG. 27 shows an example of a restricted region for non-adjacent blocks.

FIG. 28 shows a flowchart of an example method for video coding inaccordance with the presently disclosed technology.

FIG. 29 shows a flowchart of another example method for video coding inaccordance with the presently disclosed technology.

FIG. 30 shows a flowchart of yet another example method for video codingin accordance with the presently disclosed technology.

FIG. 31 shows a flowchart of a further example method for video codingin accordance with the presently disclosed technology.

FIG. 32 is a block diagram of an example of a hardware platform forimplementing a visual media decoding or a visual media encodingtechnique described in the present document.

DETAILED DESCRIPTION

Due to the increasing demand of higher resolution video, video codingmethods and techniques are ubiquitous in modern technology. Video codecstypically include an electronic circuit or software that compresses ordecompresses digital video, and are continually being improved toprovide higher coding efficiency. A video codec converts uncompressedvideo to a compressed format or vice versa. There are complexrelationships between the video quality, the amount of data used torepresent the video (determined by the bit rate), the complexity of theencoding and decoding algorithms, sensitivity to data losses and errors,ease of editing, random access, and end-to-end delay (latency). Thecompressed format usually conforms to a standard video compressionspecification, e.g., the High Efficiency Video Coding (HEVC) standard(also known as H.265 or MPEG-H Part 2), the Versatile Video Codingstandard to be finalized, or other current and/or future video codingstandards.

Embodiments of the disclosed technology may be applied to existing videocoding standards (e.g., HEVC, H.265) and future standards to improvecompression performance. Section headings are used in the presentdocument to improve readability of the description and do not in any waylimit the discussion or the embodiments (and/or implementations) to therespective sections only.

1. Example Embodiments of Video Coding

FIG. 1 shows an example block diagram of a typical HEVC video encoderand decoder. An encoding algorithm producing an HEVC compliant bitstreamwould typically proceed as follows. Each picture is split intoblock-shaped regions, with the exact block partitioning being conveyedto the decoder. The first picture of a video sequence (and the firstpicture at each clean random access point into a video sequence) iscoded using only intra-picture prediction (that uses some prediction ofdata spatially from region-to-region within the same picture, but has nodependence on other pictures). For all remaining pictures of a sequenceor between random access points, inter-picture temporally predictivecoding modes are typically used for most blocks. The encoding processfor inter-picture prediction consists of choosing motion data comprisingthe selected reference picture and motion vector (MV) to be applied forpredicting the samples of each block. The encoder and decoder generateidentical inter-picture prediction signals by applying motioncompensation (MC) using the MV and mode decision data, which aretransmitted as side information.

The residual signal of the intra- or inter-picture prediction, which isthe difference between the original block and its prediction, istransformed by a linear spatial transform. The transform coefficientsare then scaled, quantized, entropy coded, and transmitted together withthe prediction information.

The encoder duplicates the decoder processing loop (see gray-shadedboxes in FIG. 1) such that both will generate identical predictions forsubsequent data. Therefore, the quantized transform coefficients areconstructed by inverse scaling and are then inverse transformed toduplicate the decoded approximation of the residual signal. The residualis then added to the prediction, and the result of that addition maythen be fed into one or two loop filters to smooth out artifacts inducedby block-wise processing and quantization. The final picturerepresentation (that is a duplicate of the output of the decoder) isstored in a decoded picture buffer to be used for the prediction ofsubsequent pictures. In general, the order of encoding or decodingprocessing of pictures often differs from the order in which they arrivefrom the source; necessitating a distinction between the decoding order(i.e., bitstream order) and the output order (i.e., display order) for adecoder.

Video material to be encoded by HEVC is generally expected to be inputas progressive scan imagery (either due to the source video originatingin that format or resulting from deinterlacing prior to encoding). Noexplicit coding features are present in the HEVC design to support theuse of interlaced scanning, as interlaced scanning is no longer used fordisplays and is becoming substantially less common for distribution.However, a metadata syntax has been provided in HEVC to allow an encoderto indicate that interlace-scanned video has been sent by coding eachfield (i.e., the even or odd numbered lines of each video frame) ofinterlaced video as a separate picture or that it has been sent bycoding each interlaced frame as an HEVC coded picture. This provides anefficient method of coding interlaced video without burdening decoderswith a need to support a special decoding process for it.

1.1. Examples of Partition Tree Structures in H.264/AVC

The core of the coding layer in previous standards was the macroblock,containing a 16×16 block of luma samples and, in the usual case of 4:2:0color sampling, two corresponding 8×8 blocks of chroma samples.

An intra-coded block uses spatial prediction to exploit spatialcorrelation among pixels. Two partitions are defined: 16×16 and 4×4.

An inter-coded block uses temporal prediction, instead of spatialprediction, by estimating motion among pictures. Motion can be estimatedindependently for either 16×16 macroblock or any of its sub-macroblockpartitions: 16×8, 8×16, 8×8, 8×4, 4×8, 4×4, as shown in FIG. 2. Only onemotion vector (MV) per sub-macroblock partition is allowed.

1.2 Examples of Partition Tree Structures in HEVC

In HEVC, a coding tree unit (CTU) is split into coding units (CUs) byusing a quadtree structure denoted as coding tree to adapt to variouslocal characteristics. The decision whether to code a picture area usinginter-picture (temporal) or intra-picture (spatial) prediction is madeat the CU level. Each CU can be further split into one, two or fourprediction units (PUs) according to the PU splitting type. Inside onePU, the same prediction process is applied and the relevant informationis transmitted to the decoder on a PU basis. After obtaining theresidual block by applying the prediction process based on the PUsplitting type, a CU can be partitioned into transform units (TUs)according to another quadtree structure similar to the coding tree forthe CU. One of key feature of the HEVC structure is that it has themultiple partition conceptions including CU, PU, and TU.

Certain features involved in hybrid video coding using HEVC include:

-   -   (1) Coding tree units (CTUs) and coding tree block (CTB)        structure: The analogous structure in HEVC is the coding tree        unit (CTU), which has a size selected by the encoder and can be        larger than a traditional macroblock. The CTU consists of a luma        CTB and the corresponding chroma CTBs and syntax elements. The        size L×L of a luma CTB can be chosen as L=16, 32, or 64 samples,        with the larger sizes typically enabling better compression.        HEVC then supports a partitioning of the CTBs into smaller        blocks using a tree structure and quadtree-like signaling.    -   (2) Coding units (CUs) and coding blocks (CBs): The quadtree        syntax of the CTU specifies the size and positions of its luma        and chroma CBs. The root of the quadtree is associated with the        CTU. Hence, the size of the luma CTB is the largest supported        size for a luma CB. The splitting of a CTU into luma and chroma        CBs is signaled jointly. One luma CB and ordinarily two chroma        CBs, together with associated syntax, form a coding unit (CU). A        CTB may contain only one CU or may be split to form multiple        CUs, and each CU has an associated partitioning into prediction        units (PUs) and a tree of transform units (TUs).    -   (3) Prediction units and prediction blocks (PBs): The decision        whether to code a picture area using inter picture or intra        picture prediction is made at the CU level. A PU partitioning        structure has its root at the CU level. Depending on the basic        prediction-type decision, the luma and chroma CBs can then be        further split in size and predicted from luma and chroma        prediction blocks (PBs). HEVC supports variable PB sizes from        64×64 down to 4×4 samples. FIG. 3 shows examples of allowed PBs        for an M×M CU.    -   (4) Transform units (Tus) and transform blocks: The prediction        residual is coded using block transforms. A TU tree structure        has its root at the CU level. The luma CB residual may be        identical to the luma transform block (TB) or may be further        split into smaller luma TBs. The same applies to the chroma TBs.        Integer basis functions similar to those of a discrete cosine        transform (DCT) are defined for the square TB sizes 4×4, 8×8,        16×16, and 32×32. For the 4×4 transform of luma intra picture        prediction residuals, an integer transform derived from a form        of discrete sine transform (DST) is alternatively specified.        1.2.1. Examples of Tree-Structured Partitioning into TBs and TUs

For residual coding, a CB can be recursively partitioned into transformblocks (TBs). The partitioning is signaled by a residual quadtree. Onlysquare CB and TB partitioning is specified, where a block can berecursively split into quadrants, as illustrated in FIG. 4. For a givenluma CB of size M×M, a flag signals whether it is split into four blocksof size M/2×M/2. If further splitting is possible, as signaled by amaximum depth of the residual quadtree indicated in the sequenceparameter set (SPS), each quadrant is assigned a flag that indicateswhether it is split into four quadrants. The leaf node blocks resultingfrom the residual quadtree are the transform blocks that are furtherprocessed by transform coding. The encoder indicates the maximum andminimum luma TB sizes that it will use. Splitting is implicit when theCB size is larger than the maximum TB size. Not splitting is implicitwhen splitting would result in a luma TB size smaller than the indicatedminimum. The chroma TB size is half the luma TB size in each dimension,except when the luma TB size is 4×4, in which case a single 4×4 chromaTB is used for the region covered by four 4×4 luma TBs. In the case ofintra-picture-predicted CUs, the decoded samples of thenearest-neighboring TBs (within or outside the CB) are used as referencedata for intra picture prediction.

In contrast to previous standards, the HEVC design allows a TB to spanacross multiple PBs for inter-picture predicted CUs to maximize thepotential coding efficiency benefits of the quadtree-structured TBpartitioning.

1.2.2. Parent and Child Nodes

A CTB is divided according to a quad-tree structure, the nodes of whichare coding units. The plurality of nodes in a quad-tree structureincludes leaf nodes and non-leaf nodes. The leaf nodes have no childnodes in the tree structure (i.e., the leaf nodes are not furthersplit). The, non-leaf nodes include a root node of the tree structure.The root node corresponds to an initial video block of the video data(e.g., a CTB). For each respective non-root node of the plurality ofnodes, the respective non-root node corresponds to a video block that isa sub-block of a video block corresponding to a parent node in the treestructure of the respective non-root node. Each respective non-leaf nodeof the plurality of non-leaf nodes has one or more child nodes in thetree structure.

1.3. Examples of Quadtree Plus Binary Tree Block Structures with LargerCTUs in JEM

In some embodiments, future video coding technologies are explored usinga reference software known as the Joint Exploration Model (JEM). Inaddition to binary tree structures, JEM describes quadtree plus binarytree (QTBT) and ternary tree (TT) structures.

1.3.1. Examples of the QTBT Block Partitioning Structure

In contrast to HEVC, the QTBT structure removes the concepts of multiplepartition types, i.e. it removes the separation of the CU, PU and TUconcepts, and supports more flexibility for CU partition shapes. In theQTBT block structure, a CU can have either a square or rectangularshape. As shown in FIG. 5A, a coding tree unit (CTU) is firstpartitioned by a quadtree structure. The quadtree leaf nodes are furtherpartitioned by a binary tree structure. There are two splitting types,symmetric horizontal splitting and symmetric vertical splitting, in thebinary tree splitting. The binary tree leaf nodes are called codingunits (CUs), and that segmentation is used for prediction and transformprocessing without any further partitioning. This means that the CU, PUand TU have the same block size in the QTBT coding block structure. Inthe JEM, a CU sometimes consists of coding blocks (CBs) of differentcolour components, e.g. one CU contains one luma CB and two chroma CBsin the case of P and B slices of the 4:2:0 chroma format and sometimesconsists of a CB of a single component, e.g., one CU contains only oneluma CB or just two chroma CBs in the case of I slices.

The following parameters are defined for the QTBT partitioning scheme:

-   -   CTU size: the root node size of a quadtree, the same concept as        in HEVC    -   MinQTSize: the minimally allowed quadtree leaf node size    -   MaxBTSize: the maximally allowed binary tree root node size    -   MaxBTDepth: the maximally allowed binary tree depth    -   MinBTSize: the minimally allowed binary tree leaf node size

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 luma samples with two corresponding 64×64 blocks of chromasamples, the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64,the MinBTSize (for both width and height) is set as 4×4, and theMaxBTDepth is set as 4. The quadtree partitioning is applied to the CTUfirst to generate quadtree leaf nodes. The quadtree leaf nodes may havea size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size).If the leaf quadtree node is 128×128, it will not be further split bythe binary tree since the size exceeds the MaxBTSize (i.e., 64×64).Otherwise, the leaf quadtree node could be further partitioned by thebinary tree. Therefore, the quadtree leaf node is also the root node forthe binary tree and it has the binary tree depth as 0. When the binarytree depth reaches MaxBTDepth (i.e., 4), no further splitting isconsidered. When the binary tree node has width equal to MinBTSize(i.e., 4), no further horizontal splitting is considered. Similarly,when the binary tree node has height equal to MinBTSize, no furthervertical splitting is considered. The leaf nodes of the binary tree arefurther processed by prediction and transform processing without anyfurther partitioning. In the JEM, the maximum CTU size is 256×256 lumasamples.

FIG. 5A shows an example of block partitioning by using QTBT, and FIG.5B shows the corresponding tree representation. The solid lines indicatequadtree splitting and dotted lines indicate binary tree splitting. Ineach splitting (i.e., non-leaf) node of the binary tree, one flag issignalled to indicate which splitting type (i.e., horizontal orvertical) is used, where 0 indicates horizontal splitting and 1indicates vertical splitting. For the quadtree splitting, there is noneed to indicate the splitting type since quadtree splitting alwayssplits a block both horizontally and vertically to produce 4 sub-blockswith an equal size.

In addition, the QTBT scheme supports the ability for the luma andchroma to have a separate QTBT structure. Currently, for P and B slices,the luma and chroma CTBs in one CTU share the same QTBT structure.However, for I slices, the luma CTB is partitioned into CUs by a QTBTstructure, and the chroma CTBs are partitioned into chroma CUs byanother QTBT structure. This means that a CU in an I slice consists of acoding block of the luma component or coding blocks of two chromacomponents, and a CU in a P or B slice consists of coding blocks of allthree colour components.

In HEVC, inter prediction for small blocks is restricted to reduce thememory access of motion compensation, such that bi-prediction is notsupported for 4×8 and 8×4 blocks, and inter prediction is not supportedfor 4×4 blocks. In the QTBT of the JEM, these restrictions are removed.

1.4. Ternary-Tree (TT) for Versatile Video Coding (VVC)

FIG. 6A shows an example of quad-tree (QT) partitioning, and FIGS. 6Band 6C show examples of the vertical and horizontal binary-tree (BT)partitioning, respectively. In some embodiments, and in addition toquad-trees and binary-trees, ternary tree (TT) partitions, e.g.,horizontal and vertical center-side ternary-trees (as shown in FIGS. 6Dand 6E) are supported.

In some implementations, two levels of trees are supported: region tree(quad-tree) and prediction tree (binary-tree or ternary-tree). A CTU isfirstly partitioned by region tree (RT). A RT leaf may be further splitwith prediction tree (PT). A PT leaf may also be further split with PTuntil max PT depth is reached. A PT leaf is the basic coding unit. It isstill called CU for convenience. A CU cannot be further split.Prediction and transform are both applied on CU in the same way as JEM.The whole partition structure is named ‘multiple-type-tree’.

1.5. Examples of Partitioning Structures in Alternate Video CodingTechnologies

In some embodiments, a tree structure called a Multi-Tree Type (MTT),which is a generalization of the QTBT, is supported. In QTBT, as shownin FIG. 7, a Coding Tree Unit (CTU) is firstly partitioned by aquad-tree structure. The quad-tree leaf nodes are further partitioned bya binary-tree structure.

The structure of the MTT constitutes of two types of tree nodes: RegionTree (RT) and Prediction Tree (PT), supporting nine types of partitions,as shown in FIG. 8. A region tree can recursively split a CTU intosquare blocks down to a 4×4 size region tree leaf node. At each node ina region tree, a prediction tree can be formed from one of three treetypes: Binary Tree, Ternary Tree, and Asymmetric Binary Tree. In a PTsplit, it is prohibited to have a quadtree partition in branches of theprediction tree. As in JEM, the luma tree and the chroma tree areseparated in I slices.

2 Examples of Inter-Prediction in HEVC/H.265

Video coding standards have significantly improved over the years, andnow provide, in part, high coding efficiency and support for higherresolutions. Recent standards such as HEVC and H.265 are based on thehybrid video coding structure wherein temporal prediction plus transformcoding are utilized.

2.1 Examples of Prediction Modes

Each inter-predicted PU (prediction unit) has motion parameters for oneor two reference picture lists. In some embodiments, motion parametersinclude a motion vector and a reference picture index. In otherembodiments, the usage of one of the two reference picture lists mayalso be signaled using inter_pred_idc. In yet other embodiments, motionvectors may be explicitly coded as deltas relative to predictors.

When a CU is coded with skip mode, one PU is associated with the CU, andthere are no significant residual coefficients, no coded motion vectordelta or reference picture index. A merge mode is specified whereby themotion parameters for the current PU are obtained from neighboring PUs,including spatial and temporal candidates. The merge mode can be appliedto any inter-predicted PU, not only for skip mode. The alternative tomerge mode is the explicit transmission of motion parameters, wheremotion vector, corresponding reference picture index for each referencepicture list and reference picture list usage are signaled explicitlyper each PU.

When signaling indicates that one of the two reference picture lists isto be used, the PU is produced from one block of samples. This isreferred to as ‘uni-prediction’. Uni-prediction is available both forP-slices and B-slices.

When signaling indicates that both of the reference picture lists are tobe used, the PU is produced from two blocks of samples. This is referredto as ‘bi-prediction’. Bi-prediction is available for B-slices only.

2.1.1 Embodiments of Constructing Candidates for Merge Mode

When a PU is predicted using merge mode, an index pointing to an entryin the merge candidates list is parsed from the bitstream and used toretrieve the motion information. The construction of this list can besummarized according to the following sequence of steps:

Step 1: Initial candidates derivation

-   -   Step 1.1: Spatial candidates derivation    -   Step 1.2: Redundancy check for spatial candidates    -   Step 1.3: Temporal candidates derivation

Step 2: Additional candidates insertion

-   -   Step 2.1: Creation of bi-predictive candidates    -   Step 2.2: Insertion of zero motion candidates

FIG. 9 shows an example of constructing a merge candidate list based onthe sequence of steps summarized above. For spatial merge candidatederivation, a maximum of four merge candidates are selected amongcandidates that are located in five different positions. For temporalmerge candidate derivation, a maximum of one merge candidate is selectedamong two candidates. Since constant number of candidates for each PU isassumed at decoder, additional candidates are generated when the numberof candidates does not reach to maximum number of merge candidate(MaxNumMergeCand) which is signalled in slice header. Since the numberof candidates is constant, index of best merge candidate is encodedusing truncated unary binarization (TU). If the size of CU is equal to8, all the PUs of the current CU share a single merge candidate list,which is identical to the merge candidate list of the 2N×2N predictionunit.

2.1.2 Constructing Spatial Merge Candidates

In the derivation of spatial merge candidates, a maximum of four mergecandidates are selected among candidates located in the positionsdepicted in FIG. 10. The order of derivation is A₁, B₁, B₀, A₀ and B₂.Position B₂ is considered only when any PU of position A₁, B₁, B₀, A₀ isnot available (e.g. because it belongs to another slice or tile) or isintra coded. After candidate at position A₁ is added, the addition ofthe remaining candidates is subject to a redundancy check which ensuresthat candidates with same motion information are excluded from the listso that coding efficiency is improved.

To reduce computational complexity, not all possible candidate pairs areconsidered in the mentioned redundancy check. Instead only the pairslinked with an arrow in FIG. 11 are considered and a candidate is onlyadded to the list if the corresponding candidate used for redundancycheck has not the same motion information. Another source of duplicatemotion information is the “second PU” associated with partitionsdifferent from 2N×2N. As an example, FIGS. 12A and 12B depict the secondPU for the case of N×2N and 2N×N, respectively. When the current PU ispartitioned as N×2N, candidate at position A₁ is not considered for listconstruction. In some embodiments, adding this candidate may lead to twoprediction units having the same motion information, which is redundantto just have one PU in a coding unit. Similarly, position B₁ is notconsidered when the current PU is partitioned as 2N×N.

2.1.3 Constructing Temporal Merge Candidates

In this step, only one candidate is added to the list. Particularly, inthe derivation of this temporal merge candidate, a scaled motion vectoris derived based on co-located PU belonging to the picture which has thesmallest POC difference with current picture within the given referencepicture list. The reference picture list to be used for derivation ofthe co-located PU is explicitly signaled in the slice header.

FIG. 13 shows an example of the derivation of the scaled motion vectorfor a temporal merge candidate (as the dotted line), which is scaledfrom the motion vector of the co-located PU using the POC distances, tband td, where tb is defined to be the POC difference between thereference picture of the current picture and the current picture and tdis defined to be the POC difference between the reference picture of theco-located picture and the co-located picture. The reference pictureindex of temporal merge candidate is set equal to zero. For a B-slice,two motion vectors, one is for reference picture list 0 and the other isfor reference picture list 1, are obtained and combined to make thebi-predictive merge candidate.

In the co-located PU (Y) belonging to the reference frame, the positionfor the temporal candidate is selected between candidates C₀ and C₁, asdepicted in FIG. 14. If PU at position C₀ is not available, is intracoded, or is outside of the current CTU, position C₁ is used. Otherwise,position C₀ is used in the derivation of the temporal merge candidate.

2.1.4 Constructing Additional Types of Merge Candidates

Besides spatio-temporal merge candidates, there are two additional typesof merge candidates: combined bi-predictive merge candidate and zeromerge candidate. Combined bi-predictive merge candidates are generatedby utilizing spatio-temporal merge candidates. Combined bi-predictivemerge candidate is used for B-Slice only. The combined bi-predictivecandidates are generated by combining the first reference picture listmotion parameters of an initial candidate with the second referencepicture list motion parameters of another. If these two tuples providedifferent motion hypotheses, they will form a new bi-predictivecandidate.

FIG. 15 shows an example of this process, wherein two candidates in theoriginal list (710, on the left), which have mvL0 and refldxL0 or mvL1and refldxL1, are used to create a combined bi-predictive mergecandidate added to the final list (720, on the right).

Zero motion candidates are inserted to fill the remaining entries in themerge candidates list and therefore hit the MaxNumMergeCand capacity.These candidates have zero spatial displacement and a reference pictureindex which starts from zero and increases every time a new zero motioncandidate is added to the list. The number of reference frames used bythese candidates is one and two for uni- and bi-directional prediction,respectively. In some embodiments, no redundancy check is performed onthese candidates.

2.1.5 Examples of Motion Estimation Regions for Parallel Processing

To speed up the encoding process, motion estimation can be performed inparallel whereby the motion vectors for all prediction units inside agiven region are derived simultaneously. The derivation of mergecandidates from spatial neighborhood may interfere with parallelprocessing as one prediction unit cannot derive the motion parametersfrom an adjacent PU until its associated motion estimation is completed.To mitigate the trade-off between coding efficiency and processinglatency, a motion estimation region (MER) may be defined. The size ofthe MER may be signaled in the picture parameter set (PPS) using the“log 2_parallel_merge_level_minus2” syntax element. When a MER isdefined, merge candidates falling in the same region are marked asunavailable and therefore not considered in the list construction.

The picture parameter set (PPS) raw byte sequence payload (RBSP) syntaxis shown in Table 1, where log 2_parallel_merge_level_minus2 plus 2specifies the value of the variable Log 2ParMrgLevel, which is used inthe derivation process for luma motion vectors for merge mode and thederivation process for spatial merging candidates as specified in anexisting video coding standard. The value of log2parallel_merge_level_minus2 shall be in the range of 0 toCtbLog2SizeY−2, inclusive.

The variable Log 2ParMrgLevel is derived as follows:

Log 2ParMrgLevel=log 2_parallel_merge_level_minus2+2

Note that the value of Log 2ParMrgLevel indicates the built-incapability of parallel derivation of the merging candidate lists. Forexample, when Log 2ParMrgLevel is equal to 6, the merging candidatelists for all the prediction units (PUs) and coding units (CUs)contained in a 64×64 block can be derived in parallel.

TABLE 1 General picture parameter set RBSP syntax Descriptorpic_parameter_set_rbsp( ) {  pps_pic_parameter_set_id ue(v) pps_seq_parameter_set_id ue(v)  dependent_slice_segments_enabled_flagu(1) ...  pps_scaling_list_data_present_flag u(1)  if(pps_scaling_list_data_present_flag )   scaling_list_data( ) lists_modification_present_flag u(1)  log2_parallel_merge_level_minus2ue(v)  slice_segment_header_extension_present_flag u(1) pps_extension_present_flag u(1) ...  rbsp_trailing_bits( ) }

2.2 Embodiments of Motion Vector Prediction

Motion vector prediction exploits spatio-temporal correlation of motionvector with neighboring PUs, which is used for explicit transmission ofmotion parameters. It constructs a motion vector candidate list byfirstly checking availability of left, above temporally neighboring PUpositions, removing redundant candidates and adding zero vector to makethe candidate list to be constant length. Then, the encoder can selectthe best predictor from the candidate list and transmit thecorresponding index indicating the chosen candidate. Similarly withmerge index signaling, the index of the best motion vector candidate isencoded using truncated unary.

2.2.1 Examples of constructing motion vector prediction candidates

FIG. 16 summarizes derivation process for motion vector predictioncandidate, and may be implemented for each reference picture list withrefidx as an input.

In motion vector prediction, two types of motion vector candidates areconsidered: spatial motion vector candidate and temporal motion vectorcandidate. For spatial motion vector candidate derivation, two motionvector candidates are eventually derived based on motion vectors of eachPU located in five different positions as previously shown in FIG. 10.

For temporal motion vector candidate derivation, one motion vectorcandidate is selected from two candidates, which are derived based ontwo different co-located positions. After the first list ofspatio-temporal candidates is made, duplicated motion vector candidatesin the list are removed. If the number of potential candidates is largerthan two, motion vector candidates whose reference picture index withinthe associated reference picture list is larger than 1 are removed fromthe list. If the number of spatio-temporal motion vector candidates issmaller than two, additional zero motion vector candidates is added tothe list.

2.2.2 Constructing Spatial Motion Vector Candidates

In the derivation of spatial motion vector candidates, a maximum of twocandidates are considered among five potential candidates, which arederived from PUs located in positions as previously shown in FIG. 10,those positions being the same as those of motion merge. The order ofderivation for the left side of the current PU is defined as A₀, A₁, andscaled A₀, scaled A₁. The order of derivation for the above side of thecurrent PU is defined as B₀, B₁, B₂, scaled B₀, scaled B₁, scaled B₂.For each side there are therefore four cases that can be used as motionvector candidate, with two cases not required to use spatial scaling,and two cases where spatial scaling is used. The four different casesare summarized as follows:

-   -   No spatial scaling        -   (1) Same reference picture list, and same reference picture            index (same POC)        -   (2) Different reference picture list, but same reference            picture (same POC)    -   Spatial scaling        -   (3) Same reference picture list, but different reference            picture (different POC)        -   (4) Different reference picture list, and different            reference picture (different POC)

The no-spatial-scaling cases are checked first followed by the casesthat allow spatial scaling. Spatial scaling is considered when the POCis different between the reference picture of the neighbouring PU andthat of the current PU regardless of reference picture list. If all PUsof left candidates are not available or are intra coded, scaling for theabove motion vector is allowed to help parallel derivation of left andabove MV candidates. Otherwise, spatial scaling is not allowed for theabove motion vector.

As shown in the example in FIG. 17, for the spatial scaling case, themotion vector of the neighbouring PU is scaled in a similar manner asfor temporal scaling. One difference is that the reference picture listand index of current PU is given as input; the actual scaling process isthe same as that of temporal scaling.

2.2.3 Constructing Temporal Motion Vector Candidates

Apart from the reference picture index derivation, all processes for thederivation of temporal merge candidates are the same as for thederivation of spatial motion vector candidates (as shown in the examplein FIG. 14). In some embodiments, the reference picture index issignaled to the decoder.

2.2.4 Signaling of AMVP Information

For the AMVP mode, four parts may be signalled in the bitstream, e.g.,prediction direction, reference index, MVD and my predictor candidateindex, which are described in the context of the syntax shown in Table 2and Table 3.

TABLE 2 Motion vector difference syntax Descriptor prediction_unit( x0,y0, nPbW, nPbH ) {  if( cu_skip_flag[ x0 ][ y0 ] ) {   if(MaxNumMergeCand > 1 )    merge_idx[ x0 ][ y0 ] ae(v)  } else { /*MODE_INTER */   merge_flag[ x0 ][ y0 ] ae(v)   if( merge_flag[ x0 ][ y0] ) {    if( MaxNumMergeCand > 1 )     merge_idx[ x0 ][ y0 ] ae(v)   }else {    if( slice_type == B )     inter_pred_idc[ x0 ][ y0 ] ae(v)   if( inter_pred_idc[ x0 ][ y0 ] != PRED_L1 ) {     if(num_ref_idx_l0_active_minus1 > 0 )      ref_idx_l0[ x0 ][ y0 ] ae(v)    mvd_coding( x0, y0, 0 )     mvp_l0_flag[ x0 ][ y0 ] ae(v)    }   if( inter_pred_idc[ x0 ][ y0 ] != PRED_L0 ) {     if(num_ref_idx_l1_active_minus1 > 0 )      ref_idx_l1[ x0 ][ y0 ] ae(v)    if( mvd_l1_zero_flag && inter_pred_idc     [ x0 ][ y0 ] == PRED_BI ){      MvdL1[ x0 ][ y0 ][ 0 ] = 0      MvdL1[ x0 ][ y0 ][ 1 ] = 0     }else      mvd_coding( x0, y0, 1)     mvp_l1_flag[ x0 ][ y0 ] ae(v)    }  }  } }

TABLE 3 Motion vector difference semantics Descriptor mvd_coding( x0,y0, refList ) {  abs_mvd_greater0_flag[ 0 ] ae (v) abs_mvd_greater0_flag[ 1 ] ae (v)  if( abs_mvd_greater0_flag[ 0 ] )  abs_mvd_greater1_flag[ 0 ] ae (v)  if( abs_mvd_greater0_flag[ 1 ] )  abs_mvd_greater1_flag[ 1 ] ae (v)  if( abs_mvd_greater0_flag[ 0 ] ) {  if( abs_mvd_greater1_flag[ 0 ] )    abs_mvd_minus2[ 0 ] ae (v)  mvd_sign_flag[ 0 ] ae (v)  }  if( abs_mvd_greater0_flag[ 1 ] ) {   if(abs_mvd_greater1_flag[ 1 ] )    abs_mvd_minus2[ 1 ] ae (v)  mvd_sign_flag[ 1 ] ae (v)  } }

3 Examples of Inter Prediction Methods in Joint Exploration Model (JEM)

In some embodiments, future video coding technologies are explored usinga reference software known as the Joint Exploration Model (JEM). In JEM,sub-block based prediction is adopted in several coding tools, such asaffine prediction, alternative temporal motion vector prediction(ATMVP), spatial-temporal motion vector prediction (STMVP),bi-directional optical flow (BIO), Frame-Rate Up Conversion (FRUC),Locally Adaptive Motion Vector Resolution (LAMVR), Overlapped BlockMotion Compensation (OBMC), Local Illumination Compensation (LIC), andDecoder-side Motion Vector Refinement (DMVR).

3.1 Examples of Sub-CU Based Motion Vector Prediction

In the JEM with quadtrees plus binary trees (QTBT), each CU can have atmost one set of motion parameters for each prediction direction. In someembodiments, two sub-CU level motion vector prediction methods areconsidered in the encoder by splitting a large CU into sub-CUs andderiving motion information for all the sub-CUs of the large CU.Alternative temporal motion vector prediction (ATMVP) method allows eachCU to fetch multiple sets of motion information from multiple blockssmaller than the current CU in the collocated reference picture. Inspatial-temporal motion vector prediction (STMVP) method motion vectorsof the sub-CUs are derived recursively by using the temporal motionvector predictor and spatial neighbouring motion vector. In someembodiments, and to preserve more accurate motion field for sub-CUmotion prediction, the motion compression for the reference frames maybe disabled.

3.1.1 Examples of Alternative Temporal Motion Vector Prediction (ATMVP)

In the ATMVP method, the temporal motion vector prediction (TMVP) methodis modified by fetching multiple sets of motion information (includingmotion vectors and reference indices) from blocks smaller than thecurrent CU.

FIG. 18 shows an example of ATMVP motion prediction process for a CU1800. The ATMVP method predicts the motion vectors of the sub-CUs 1801within a CU 1800 in two steps. The first step is to identify thecorresponding block 1851 in a reference picture 1850 with a temporalvector. The reference picture 1850 is also referred to as the motionsource picture. The second step is to split the current CU 1800 intosub-CUs 1801 and obtain the motion vectors as well as the referenceindices of each sub-CU from the block corresponding to each sub-CU.

In the first step, a reference picture 1850 and the corresponding blockis determined by the motion information of the spatial neighboringblocks of the current CU 1800. To avoid the repetitive scanning processof neighboring blocks, the first merge candidate in the merge candidatelist of the current CU 1800 is used. The first available motion vectoras well as its associated reference index are set to be the temporalvector and the index to the motion source picture. This way, thecorresponding block may be more accurately identified, compared withTMVP, wherein the corresponding block (sometimes called collocatedblock) is always in a bottom-right or center position relative to thecurrent CU.

In one example, if the first merge candidate is from the leftneighboring block (i.e., Ai in FIG. 19), the associated MV and referencepicture are utilized to identify the source block and source picture.

In the second step, a corresponding block of the sub-CU 1851 isidentified by the temporal vector in the motion source picture 1850, byadding to the coordinate of the current CU the temporal vector. For eachsub-CU, the motion information of its corresponding block (e.g., thesmallest motion grid that covers the center sample) is used to derivethe motion information for the sub-CU. After the motion information of acorresponding N×N block is identified, it is converted to the motionvectors and reference indices of the current sub-CU, in the same way asTMVP of HEVC, wherein motion scaling and other procedures apply. Forexample, the decoder checks whether the low-delay condition (e.g. thePOCs of all reference pictures of the current picture are smaller thanthe POC of the current picture) is fulfilled and possibly uses motionvector MVx (e.g., the motion vector corresponding to reference picturelist X) to predict motion vector MVy (e.g., with X being equal to 0 or 1and Y being equal to 1−X) for each sub-CU.

3.1.2 Examples of Spatial-Temporal Motion Vector Prediction (STMVP)

In the STMVP method, the motion vectors of the sub-CUs are derivedrecursively, following raster scan order. FIG. 20 shows an example ofone CU with four sub-blocks and neighboring blocks. Consider an 8×8 CU2000 that includes four 4×4 sub-CUs A (2001), B (2002), C (2003), and D(2004). The neighboring 4×4 blocks in the current frame are labelled asa (2011), b (2012), c (2013), and d (2014).

The motion derivation for sub-CU A starts by identifying its two spatialneighbors. The first neighbor is the N×N block above sub-CU A 1101(block c 2013). If this block c (2013) is not available or is intracoded the other N×N blocks above sub-CU A (2001) are checked (from leftto right, starting at block c 2013). The second neighbor is a block tothe left of the sub-CU A 2001 (block b 2012). If block b (2012) is notavailable or is intra coded other blocks to the left of sub-CU A 2001are checked (from top to bottom, staring at block b 2012). The motioninformation obtained from the neighboring blocks for each list is scaledto the first reference frame for a given list. Next, temporal motionvector predictor (TMVP) of sub-block A 2001 is derived by following thesame procedure of TMVP derivation as specified in HEVC. The motioninformation of the collocated block at block D 2004 is fetched andscaled accordingly. Finally, after retrieving and scaling the motioninformation, all available motion vectors are averaged separately foreach reference list. The averaged motion vector is assigned as themotion vector of the current sub-CU.

3.1.3 Examples of Sub-CU Motion Prediction Mode Signaling

In some embodiments, the sub-CU modes are enabled as additional mergecandidates and there is no additional syntax element required to signalthe modes. Two additional merge candidates are added to merge candidateslist of each CU to represent the ATMVP mode and STMVP mode. In otherembodiments, up to seven merge candidates may be used, if the sequenceparameter set indicates that ATMVP and STMVP are enabled. The encodinglogic of the additional merge candidates is the same as for the mergecandidates in the HM, which means, for each CU in P or B slice, two moreRD checks may be needed for the two additional merge candidates. In someembodiments, e.g., JEM, all bins of the merge index are context coded byCABAC (Context-based Adaptive Binary Arithmetic Coding). In otherembodiments, e.g., HEVC, only the first bin is context coded and theremaining bins are context by-pass coded.

3.2 Examples of Adaptive Motion Vector Difference Resolution

In some embodiments, motion vector differences (MVDs) (between themotion vector and predicted motion vector of a PU) are signalled inunits of quarter luma samples when use_integer_mv_flag is equal to 0 inthe slice header. In the JEM, a locally adaptive motion vectorresolution (LAMVR) is introduced. In the JEM, MVD can be coded in unitsof quarter luma samples, integer luma samples or four luma samples. TheMVD resolution is controlled at the coding unit (CU) level, and MVDresolution flags are conditionally signalled for each CU that has atleast one non-zero MVD components.

For a CU that has at least one non-zero MVD components, a first flag issignalled to indicate whether quarter luma sample MV precision is usedin the CU. When the first flag (equal to 1) indicates that quarter lumasample MV precision is not used, another flag is signalled to indicatewhether integer luma sample MV precision or four luma sample MVprecision is used.

When the first MVD resolution flag of a CU is zero, or not coded for aCU (meaning all MVDs in the CU are zero), the quarter luma sample MVresolution is used for the CU. When a CU uses integer-luma sample MVprecision or four-luma-sample MV precision, the MVPs in the AMVPcandidate list for the CU are rounded to the corresponding precision.

In the encoder, CU-level RD checks are used to determine which MVDresolution is to be used for a CU. That is, the CU-level RD check isperformed three times for each MVD resolution. To accelerate encoderspeed, the following encoding schemes are applied in the JEM:

-   -   During RD check of a CU with normal quarter luma sample MVD        resolution, the motion information of the current CU (integer        luma sample accuracy) is stored. The stored motion information        (after rounding) is used as the starting point for further small        range motion vector refinement during the RD check for the same        CU with integer luma sample and 4 luma sample MVD resolution so        that the time-consuming motion estimation process is not        duplicated three times.    -   RD check of a CU with 4 luma sample MVD resolution is        conditionally invoked. For a CU, when RD cost integer luma        sample MVD resolution is much larger than that of quarter luma        sample MVD resolution, the RD check of 4 luma sample MVD        resolution for the CU is skipped.

3.3 Examples of Pattern Matched Motion Vector Derivation (PMMVD)

The PMMVD mode is a special merge mode based on the Frame-Rate UpConversion (FRUC) method. With this mode, motion information of a blockis not signaled but derived at decoder side.

A FRUC flag can be signaled for a CU when its merge flag is true. Whenthe FRUC flag is false, a merge index can be signaled and the regularmerge mode is used. When the FRUC flag is true, an additional FRUC modeflag can be signaled to indicate which method (e.g., bilateral matchingor template matching) is to be used to derive motion information for theblock.

At the encoder side, the decision on whether using FRUC merge mode for aCU is based on RD cost selection as done for normal merge candidate. Forexample, multiple matching modes (e.g., bilateral matching and templatematching) are checked for a CU by using RD cost selection. The oneleading to the minimal cost is further compared to other CU modes. If aFRUC matching mode is the most efficient one, FRUC flag is set to truefor the CU and the related matching mode is used.

Typically, motion derivation process in FRUC merge mode has two steps: aCU-level motion search is first performed, then followed by a Sub-CUlevel motion refinement. At CU level, an initial motion vector isderived for the whole CU based on bilateral matching or templatematching. First, a list of MV candidates is generated and the candidatethat leads to the minimum matching cost is selected as the startingpoint for further CU level refinement. Then a local search based onbilateral matching or template matching around the starting point isperformed. The MV results in the minimum matching cost is taken as theMV for the whole CU. Subsequently, the motion information is furtherrefined at sub-CU level with the derived CU motion vectors as thestarting points.

For example, the following derivation process is performed for a W×H CUmotion information derivation. At the first stage, MV for the whole W×HCU is derived. At the second stage, the CU is further split into M×Msub-CUs. The value of M is calculated as in Eq. (1), D is a predefinedsplitting depth which is set to 3 by default in the JEM. Then the MV foreach sub-CU is derived.

$\begin{matrix}{M = {\max\left\{ {4,{\min\left\{ {\frac{M}{2^{D}},\frac{N}{2^{D}}} \right\}}} \right\}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$

FIG. 21 shows an example of bilateral matching used in the Frame-Rate UpConversion (FRUC) method. The bilateral matching is used to derivemotion information of the current CU by finding the closest matchbetween two blocks along the motion trajectory of the current CU (2100)in two different reference pictures (2110, 2111). Under the assumptionof continuous motion trajectory, the motion vectors MV0 (2101) and MV1(2102) pointing to the two reference blocks are proportional to thetemporal distances, e.g., TD0 (2103) and TD1 (2104), between the currentpicture and the two reference pictures. In some embodiments, when thecurrent picture 2100 is temporally between the two reference pictures(2110, 2111) and the temporal distance from the current picture to thetwo reference pictures is the same, the bilateral matching becomesmirror based bi-directional MV.

FIG. 22 shows an example of template matching used in the Frame-Rate UpConversion (FRUC) method. Template matching can be used to derive motioninformation of the current CU 2200 by finding the closest match betweena template (e.g., top and/or left neighboring blocks of the current CU)in the current picture and a block (e.g., same size to the template) ina reference picture 2210. Except the aforementioned FRUC merge mode, thetemplate matching can also be applied to AMVP mode. In both JEM andHEVC, AMVP has two candidates. With the template matching method, a newcandidate can be derived. If the newly derived candidate by templatematching is different to the first existing AMVP candidate, it isinserted at the very beginning of the AMVP candidate list and then thelist size is set to two (e.g., by removing the second existing AMVPcandidate). When applied to AMVP mode, only CU level search is applied.

The MV candidate set at CU level can include the following: (1) originalAMVP candidates if the current CU is in AMVP mode, (2) all mergecandidates, (3) several MVs in the interpolated MV field (describedlater), and top and left neighboring motion vectors.

When using bilateral matching, each valid MV of a merge candidate can beused as an input to generate a MV pair with the assumption of bilateralmatching. For example, one valid MV of a merge candidate is (MVa,ref_(a)) at reference list A. Then the reference picture ref_(b) of itspaired bilateral MV is found in the other reference list B so thatref_(a) and ref_(b) are temporally at different sides of the currentpicture. If such a ref_(b) is not available in reference list B, ref_(b)is determined as a reference which is different from ref_(a) and itstemporal distance to the current picture is the minimal one in list B.After ref_(b) is determined, MVb is derived by scaling MVa based on thetemporal distance between the current picture and ref_(a), ref_(b).

In some implementations, four MVs from the interpolated MV field canalso be added to the CU level candidate list. More specifically, theinterpolated MVs at the position (0, 0), (W/2, 0), (0, H/2) and (W/2,H/2) of the current CU are added. When FRUC is applied in AMVP mode, theoriginal AMVP candidates are also added to CU level MV candidate set. Insome implementations, at the CU level, 15 MVs for AMVP CUs and 13 MVsfor merge CUs can be added to the candidate list.

The MV candidate set at sub-CU level includes an MV determined from aCU-level search, (2) top, left, top-left and top-right neighboring MVs,(3) scaled versions of collocated MVs from reference pictures, (4) oneor more ATMVP candidates (e.g., up to four), and (5) one or more STMVPcandidates (e.g., up to four). The scaled MVs from reference picturesare derived as follows. The reference pictures in both lists aretraversed. The MVs at a collocated position of the sub-CU in a referencepicture are scaled to the reference of the starting CU-level MV. ATMVPand STMVP candidates can be the four first ones. At the sub-CU level,one or more MVs (e.g., up to 17) are added to the candidate list.

Generation of an interpolated MV field. Before coding a frame,interpolated motion field is generated for the whole picture based onunilateral ME. Then the motion field may be used later as CU level orsub-CU level MV candidates.

In some embodiments, the motion field of each reference pictures in bothreference lists is traversed at 4×4 block level. FIG. 23 shows anexample of unilateral Motion Estimation (ME) 2300 in the FRUC method.For each 4×4 block, if the motion associated to the block passingthrough a 4×4 block in the current picture and the block has not beenassigned any interpolated motion, the motion of the reference block isscaled to the current picture according to the temporal distance TD0 andTD1 (the same way as that of MV scaling of TMVP in HEVC) and the scaledmotion is assigned to the block in the current frame. If no scaled MV isassigned to a 4×4 block, the block's motion is marked as unavailable inthe interpolated motion field.

Interpolation and matching cost. When a motion vector points to afractional sample position, motion compensated interpolation is needed.To reduce complexity, bi-linear interpolation instead of regular 8-tapHEVC interpolation can be used for both bilateral matching and templatematching.

The calculation of matching cost is a bit different at different steps.When selecting the candidate from the candidate set at the CU level, thematching cost can be the absolute sum difference (SAD) of bilateralmatching or template matching. After the starting MV is determined, thematching cost C of bilateral matching at sub-CU level search iscalculated as follows:

C=SAD+w·(|MV _(x) −MV _(x) ^(s) |+|MV _(y) −MV _(y) ^(s)|)  Eq. (2)

Here, w is a weighting factor. In some embodiments, w can be empiricallyset to 4. MV and MV^(s) indicate the current MV and the starting MV,respectively. SAD may still be used as the matching cost of templatematching at sub-CU level search.

In FRUC mode, MV is derived by using luma samples only. The derivedmotion will be used for both luma and chroma for MC inter prediction.After MV is decided, final MC is performed using 8-taps interpolationfilter for luma and 4-taps interpolation filter for chroma.

MV refinement is a pattern based MV search with the criterion ofbilateral matching cost or template matching cost. In the JEM, twosearch patterns are supported—an unrestricted center-biased diamondsearch (UCBDS) and an adaptive cross search for MV refinement at the CUlevel and sub-CU level, respectively. For both CU and sub-CU level MVrefinement, the MV is directly searched at quarter luma sample MVaccuracy, and this is followed by one-eighth luma sample MV refinement.The search range of MV refinement for the CU and sub-CU step are setequal to 8 luma samples.

In the bilateral matching merge mode, bi-prediction is applied becausethe motion information of a CU is derived based on the closest matchbetween two blocks along the motion trajectory of the current CU in twodifferent reference pictures. In the template matching merge mode, theencoder can choose among uni-prediction from list0, uni-prediction fromlist1, or bi-prediction for a CU. The selection ca be based on atemplate matching cost as follows:

If costBi <= factor * min (cost0, cost1) bi-prediction is used;Otherwise, if cost0 <= cost1 uni-prediction from list0 is used;Otherwise, uni-prediction from list1 is used;

Here, cost0 is the SAD of list0 template matching, cost1 is the SAD oflist1 template matching and costBi is the SAD of bi-prediction templatematching. For example, when the value of factor is equal to 1.25, itmeans that the selection process is biased toward bi-prediction. Theinter prediction direction selection can be applied to the CU-leveltemplate matching process.

3.4 Examples of Decoder-Side Motion Vector Refinement (DMVR)

In a bi-prediction operation, for the prediction of one block region,two prediction blocks, formed using a motion vector (MV) of list0 and aMV of list1, respectively, are combined to form a single predictionsignal. In the decoder-side motion vector refinement (DMVR) method, thetwo motion vectors of the bi-prediction are further refined by abilateral template matching process. The bilateral template matchingapplied in the decoder to perform a distortion-based search between abilateral template and the reconstruction samples in the referencepictures in order to obtain a refined MV without transmission ofadditional motion information.

In DMVR, a bilateral template is generated as the weighted combination(i.e. average) of the two prediction blocks, from the initial MV0 oflist0 and MV1 of list1, respectively, as shown in FIG. 24. The templatematching operation consists of calculating cost measures between thegenerated template and the sample region (around the initial predictionblock) in the reference picture. For each of the two reference pictures,the MV that yields the minimum template cost is considered as theupdated MV of that list to replace the original one. In the JEM, nine MVcandidates are searched for each list. The nine MV candidates includethe original MV and 8 surrounding MVs with one luma sample offset to theoriginal MV in either the horizontal or vertical direction, or both.Finally, the two new MVs, i.e., MV0′ and MV1′ as shown in FIG. 24, areused for generating the final bi-prediction results. A sum of absolutedifferences (SAD) is used as the cost measure.

DMVR is applied for the merge mode of bi-prediction with one MV from areference picture in the past and another from a reference picture inthe future, without the transmission of additional syntax elements. Inthe JEM, when LIC, affine motion, FRUC, or sub-CU merge candidate isenabled for a CU, DMVR is not applied.

3.5 Examples of Merge/Skip Mode with Bilateral Matching Refinement

A merge candidate list is first constructed by inserting the motionvectors and reference indices of the spatial neighboring and temporalneighboring blocks into the candidate list with redundancy checkinguntil the number of the available candidates reaches the maximumcandidate size of 19. The merge candidate list for the merge/skip modeis constructed by inserting spatial candidates, temporal candidates,affine candidates, advanced temporal MVP (ATMVP) candidate, spatialtemporal MVP (STMVP) candidate and the additional candidates as used inHEVC (Combined candidates and Zero candidates) according to apre-defined insertion order, and in the context of the numbered blocksshown in FIG. 25:

-   -   (1) Spatial candidates for blocks 1-4    -   (2) Extrapolated affine candidates for blocks 1-4    -   (3) ATMVP    -   (4) STMVP    -   (5) Virtual affine candidate    -   (6) Spatial candidate (block 5) (used only when the number of        the available candidates is smaller than 6)    -   (7) Extrapolated affine candidate (block 5)    -   (8) Temporal candidate (derived as in HEVC)    -   (9) Non-adjacent spatial candidate followed by extrapolated        affine candidate (blocks 6 to 49)    -   (10) Combined candidates    -   (11) Zero candidates

It may be noted that IC flags are also inherited from merge candidatesexcept for STMVP and affine. Moreover, for the first four spatialcandidates, the bi-prediction ones are inserted before the ones withuni-prediction.

3.5.1 Non-Adjacent Merge Candidates

Non-adjacent merge candidates may be added to the merge candidate listif the total number of available merge candidates hasn't reached themaximumly allowed merge candidates. In an existing implementation,non-adjacent merge candidates may be inserted to the merge candidatelist after the TMVP merge candidate. The process of adding non-adjacentmerge candidates could be performed by the pseudocode shown in FIG. 26.

4. Examples of Existing Implementations

In existing implementations, using non-adjacent merge candidates whichobtain motion information from non-adjacent blocks may result insuboptimal performance.

In one example, prediction from motion information of non-adjacentblocks located above CTU rows may significantly increase the line buffersize.

In another example, prediction from motion information of non-adjacentblocks could bring additional coding gain with the cost of storing allthe motion information (typically on 4×4 level) into cache whichsignificantly increase the complexity for hardware implementation.

5. Example Methods for Constructing Non-Adjacent Merge Candidates

Embodiments of the presently disclosed technology overcome the drawbacksof existing implementations, thereby providing video coding with lowermemory and complexity requirements and higher coding efficiencies. Theselection of non-adjacent merge candidates based on the disclosedtechnology, which may enhance both existing and future video codingstandards, is elucidated in the following examples described for variousimplementations. The examples of the disclosed technology provided belowexplain general concepts, and are not meant to be interpreted aslimiting. In an example, unless explicitly indicated to the contrary,the various features described in these examples may be combined.

Embodiments of the disclosed technology reduce the cache/line buffersize required by the non-adjacent merge candidate as well as methods forfurther improving the coding performance of non-adjacent mergecandidates.

For the examples discussed below, let the top-left sample coordinate ofthe current block be (Cx, Cy) and the coordinate of the top-left samplein one non-adjacent block by (NAx, NAy), with the origin (0,0) to be thetop-left point of a picture/slice/tile/LCU row/LCU. The coordinatedifference (i.e., offset from the current block) is denoted by (offsetX,offsetY) wherein offsetX=Cx−NAx and offsetY=Cy−NAy.

Examples 1 and 2 advantageously provide, at least, memory and bufferreductions.

Example 1. In one example, only non-adjacent blocks located at certainpositions are accessed when constructing merge candidates.

-   -   (a) In one example, x and y should satisfy that NAx % M=0 and        NAy % N=0 wherein M and N are two non-zero integers, such as        M=N=8 or 16.    -   (b) In one example, if the top-left sample in one non-adjacent        block doesn't satisfy the given conditions, the checking of the        motion information associated with this block is skipped.        Therefore, the associated motion information couldn't be added        to the merge candidate list.    -   (c) Alternatively, if the top-left sample in one non-adjacent        block doesn't satisfy the given conditions, the position of this        block may be shifted, truncated or rounded to make sure the        conditions are satisfied. For example, (NAx, NAy) may be        modified to ((NAx/M)*M, (NAy/N)*N) wherein ‘/’ is the integer        division.    -   (d) The restricted region size covering all the non-adjacent        blocks may be pre-defined/signaled. In this case, when a        non-adjacent block calculated by a given offset (OffsetX,        OffsetY) is outside the region, it is marked as unavailable or        treated as intra-code mode. The corresponding motion information        could be added a candidate to the candidate list. An example is        depicted in FIG. 27.        -   (i) In one example, the region size is defined as one or            more CTBs.        -   (ii) Alternatively, all blocks above the LCU row covering            the current block are marked as unavailable or treated as            intra-code mode. The corresponding motion information could            be added a candidate to the candidate list.        -   (iii) Alternatively, suppose the top-left sample coordinate            of the LCU covering current block by (LX, LY). (LX−NAx)            and/or abs(LX−NAx) and/or (LY−NAy) and/or abs(LY−NAy) should            be within a threshold.        -   (iv) One or multiple thresholds may be predefined. They            could be further dependent on minimum size of CU            height/minimum size of width/LCU size etc., al. For example,            (LY−NAy) should be less than the minimum size of CU height,            or (LY−NAy) should be less than twice of the minimum size of            CU height.        -   (v) The region size or the threshold(s) may be signaled in a            Sequence Parameter Set (SPS), a Picture Parameter Set (PPS),            a Video Parameter Set (VPS), a slice header, a tile header,            and so on.        -   (vi) In one example, all non-adjacent blocks outside the            current slice/tile/other kinds of unit for parallel coding            are marked as unavailable, and the corresponding motion            information should not be added as a candidate to the            candidate list.

Example 2. Spatial motion compression may be applied aftercoding/decoding one region (e.g., CTU, multiple CTUs).

-   -   (a) When motion compression is applied to a region, it means        that all blocks within the region are forced to share one        representative set of motion information.    -   (b) After coding/decoding a region, spatial motion compression        may be applied.    -   (c) Alternatively, furthermore, the representative set of motion        information is utilized to predict the following blocks.    -   (d) Alternatively, furthermore, the spatial motion compression        is applied before filtering process, e.g., deblocking filtering,        Sample Adaptive Offset (SAO) filtering.    -   (e) Alternatively, furthermore, the compressed motion        information are used in the filtering process.    -   (f) In one example, the set of motion information from a certain        (e.g., the center or bottom-right) position of the region is        selected as the representative set.    -   (g) Alternatively, multiple positions may be checked in order,        once an inter-coded block for a position is identified, the        checking process terminates and the associated motion        information is used as the representative set.    -   (h) In one example, a set of motion information may be derived        from the motion information associated with the region and used        as the representative set.

Example 3 advantageously provides, at least, reduction in computationalcomplexity.

Example 3. When inserting a new non-adjacent merge candidate, pruningmay be applied to partial of available merge candidates.

-   -   (a) In one example, the new non-adjacent merge candidate is not        pruned to other inserted non-adjacent merge candidates.    -   (b) In one example, the new non-adjacent merge candidate is not        pruned to the temporal merge candidate, such as TMVP or ATMVP.    -   (c) In one example, the new non-adjacent merge candidate is        pruned with some merge candidates from some specific neighboring        blocks, but not pruned with some other merge candidates from        some other specific neighboring blocks.

Examples 4-7 advantageously provide, at least, coding performance gains.

Example 4. Not all kinds of motion information of non-adjacent blocksare considered in the merge candidate list.

-   -   (a) In one example, only motion information of AMVP-coded        non-adjacent blocks are considered.    -   (b) In one example, only motion information of merge-coded        non-adjacent blocks are considered.    -   (c) In one example, only motion information of merge-coded        non-adjacent blocks and not from a spatial merge candidate are        considered.    -   (d) In one example, only motion information of merge-coded        non-adjacent blocks with motion refinement process invoked are        considered.    -   (e) In one example, motion information of non-adjacent blocks        coded with any kind of decoder side MV refinement, such as FRUC,        template-matching, or PMVR, are not considered.    -   (f) In one example, motion information with low MVD/MV        precision, like 4-pel precision, are not considered.

Example 5. For the non-adjacent spatial neighbors, those coded with afirst mode are given higher priority compared to those coded with asecond mode (e.g., checked before all the non-adjacent spatial neighborscoded with the second mode).

-   -   (a) Alternatively, for each distance, non-adjacent spatial        neighbors coded with the first modes are given higher priority        compared to those coded with the second mode. While for        different distances, non-adjacent spatial neighbors with smaller        distance are given higher priority.    -   (b) In one example, the first mode is defined as the AMVP mode,        and the second mode is defined as the merge mode, and vice        versa.    -   (c) Alternatively, the first mode is defined as the        bi-prediction mode, and the second mode is defined as the        uni-prediction mode, and vice versa.    -   (d) Alternatively, the first mode is defined as the AMVP mode,        and the second mode is defined as the merge mode but not a        spatial merge candidate.    -   (e) Alternatively, the first mode is defined as affine mode, and        the second is defined as non-affine mode, or vice versa.    -   (f) Alternatively, the first mode is defined as high MV        precision mode, and the second mode is defined as low MV        precision mode.

Example 6. Instead of checking non-adjacent spatial neighbors after acertain kind of merge candidate (e.g., TMVP), multiple stages ofnon-adjacent merge candidates may be derived and added to mergecandidate list.

-   -   (a) In one example, for the first stage, those non-adjacent        spatial neighbors coded with a first mode may be added after the        first given kind of merge candidate. While for the second stage,        those non-adjacent spatial neighbors coded with a second mode        may be added after the second given kind of merge candidate, and        so on.    -   (b) In one example, the first given kind of merge candidate may        be the last spatial merge candidate while the second given kind        of merge candidate may be the TMVP merge candidate.    -   (c) Alternatively, furthermore, the multiple stages may involve        different positions of non-adjacent blocks.    -   (d) The first and second modes may be defined as in Example 5.

Example 7. The motion information of non-adjacent blocks may be used apredictor in the AMVP mode.

-   -   (a) In one example, one or multiple MV candidates derived (e.g.,        scaling to the target reference picture)/inherited from one or        multiple non-adjacent blocks may be added to the AMVP candidate        list.    -   (b) Alternatively, one or multiple motion candidates inherited        from one or multiple non-adjacent blocks may be added to the        AMVP candidate list. In this case, the signaling of reference        picture index as well as prediction directions are skipped, and        they are inherited from the motion candidate.    -   (c) The usage of motion information associated with non-adjacent        blocks may be applied under certain conditions, e.g., the total        number of AMVP candidates doesn't reach the maximumly allowed        size, and/or when the indication of the usage is signaled,        and/or the block size (W, H) where W>th0 and H>th1 (th0 and th1        are two thresholds, e.g., both equal to 8).

FIG. 28 shows a flowchart of an exemplary method for video coding, whichmay be implemented in a video encoder. The method 2800 includes, at step2810, receiving a current block of video data. The method 2800 includes,at step 2820, selecting, based on a rule, a first non-adjacent blockthat is not adjacent to the current block. The method 2800 includes, atstep 2830, constructing a first merge candidate comprising motioninformation based on the first non-adjacent block. The method 2800includes, at step 2840, identifying a second non-adjacent block that isnot adjacent to the current block and different from the firstnon-adjacent block. The method 2800 includes, at step 2850, based ondetermining that the second non-adjacent block fails to satisfy therule, refraining adding a second merge candidate derived from the secondnon-adjacent block. The method 2800 includes, at step 2860, constructinga merge candidate list based on the first non-adjacent block. The method2800 includes, at step 2870, decoding the current block based on themotion information.

FIG. 29 shows a flowchart of another example method for video coding inaccordance with the presently disclosed technology. The method 2900includes, at step 2910, receiving a current block of video data. Themethod 2900 includes, at step 2920, selecting, based on a rule, a firstnon-adjacent block that is not adjacent to the current block. The method2900 includes, at step 2930, constructing a first merge candidatecomprising motion information based on the first non-adjacent block. Themethod 2900 includes, at step 2940, selecting a second non-adjacentblock, wherein the first non-adjacent block and the second non-adjacentblock are not adjacent to the current block, wherein the firstnon-adjacent block is coded with a first mode, wherein the secondnon-adjacent block is coded with a second mode, and wherein the firstnon-adjacent block is selected prior to selecting the secondnon-adjacent block. The method 2900 includes, at step 2950, constructinga second merge candidate comprising motion information based on thesecond non-adjacent block.

FIG. 30 shows a flowchart of yet another example method for video codingin accordance with the presently disclosed technology. The method 3000includes, at step 3010, receiving a current block of video data. Themethod 3000 includes, at step 3020, selecting, based on rules, a firstsubset of non-adjacent blocks and a second subset of non-adjacent blocksthat are not adjacent to the current block. The method 3000 includes, atstep 3030, constructing a first subset of merge candidates comprisingmotion information based on the first subset of non-adjacent blocks anda second subset of merge candidates comprising motion information basedon the second subset of non-adjacent blocks, wherein the first subset ofmerge candidates is added into a merge candidate list after a mergecandidate of a first type, and wherein the second subset of mergecandidates is added into the merge candidate list after a mergecandidate of a second type.

FIG. 31 shows a flowchart of a further example method for video codingin accordance with the presently disclosed technology. The method 3100includes, at step 3110, receiving a current block of video data. Themethod includes, at step 3120, selecting, based on a rule, a firstnon-adjacent block that is not adjacent to the current block. The methodincludes, at step 3130, Constructing at least one motion vectorpredictor candidate based on the first non-adjacent block, wherein theat least one motion vector predictor candidate is used as a predictor inan advanced motion vector prediction (AMVP) mode. The method includes,at step 3140, decoding a block associated with the AMVP mode based onthe at least one motion vector predictor candidate.

Some features preferred by some embodiments are provided in clause-basedformat as follows. These embodiments include further variations andaspect of techniques described in Examples 4 to 7.

1. A method for video processing (e.g., method 2800), comprising:receiving a current block of video data; selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block;constructing a first merge candidate comprising motion information basedon the first non-adjacent block; identifying a second non-adjacent blockthat is not adjacent to the current block and different from the firstnon-adjacent block; based on determining that the second non-adjacentblock fails to satisfy the rule, refraining adding a second mergecandidate derived from the second non-adjacent block; constructing amerge candidate list based on the first non-adjacent block; and decodingthe current block based on the merge candidate list.

2. The method of clause 1, wherein the rule indicates a codingcharacteristic of the first non-adjacent block and the secondnon-adjacent block.

3. The method of clause 2, wherein the rule indicates that the firstnon-adjacent block is an advanced motion vector prediction (AMVP)-codednon-adjacent block.

4. The method of clause 2, wherein the rule indicates that the firstnon-adjacent block is a merge-coded non-adjacent block.

5. The method of clause 4, wherein the rule indicates that the firstnon-adjacent block is not coded with a spatial merge candidate.

6. The method of clause 4, wherein the rule indicates that the firstnon-adjacent block is coded with a motion refinement process.

7. The method of clause 2, wherein the rule indicates that the firstnon-adjacent is not coded with decoder-side motion vector refinement(DMVR).

8. The method of clause 2, wherein the rule indicates that the firstnon-adjacent block is coded with a type of motion information thatexcludes motion information with low precision motion vectors or motionvector differences.

9. A method for video processing (e.g., method 2900), comprising:receiving a current block of video data; selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block;constructing a first merge candidate comprising motion information basedon the first non-adjacent block; selecting a second non-adjacent block,wherein the first non-adjacent block and the second non-adjacent blockare not adjacent to the current block, wherein the first non-adjacentblock is coded with a first mode, and the second non-adjacent block iscoded with a second mode, and wherein the first non-adjacent block isselected prior to selecting the second non-adjacent block; andconstructing a second merge candidate comprising motion informationbased on the second non-adjacent block.

10. The method of clause 9, further comprising: constructing a mergecandidate list based on the first merge candidate and the second mergecandidate.

11. The method of any one or more of clauses 10, wherein the secondmerge candidate is added to the merge candidate list after the firstmerge candidate.

12. The method of clause 9, wherein the first mode is an advanced motionvector prediction (AMVP) mode and the second mode is a merge mode, orvice versa.

13. The method of clause 12, wherein the merge mode is associated with amerge index which does not correspond to a spatial merge candidate.

14. The method of clause 9, wherein the first mode is a bi-predictionmode and the second mode is a uni-prediction mode, or vice versa.

15. The method of clause 9, wherein the first mode is an affine mode andthe second mode is a non-affine mode, or vice versa.

16. The method of clause 9, wherein the first mode is a high motionvector (MV) precision mode and the second mode is a low MV precisionmode.

17. A method for video processing (e.g., method 3000), comprising:receiving a current block of video data; selecting, based on rules, afirst subset of non-adjacent blocks and a second subset of non-adjacentblocks that are not adjacent to the current block; and constructing afirst subset of merge candidates comprising motion information based onthe first subset of non-adjacent blocks and a second subset of mergecandidates comprising motion information based on the second subset ofnon-adjacent blocks, wherein the first subset of merge candidates isadded into a merge candidate list after a merge candidate of a firsttype, and wherein the second subset of merge candidates is added intothe merge candidate list after a merge candidate of a second type.

18. The method of clause 17, wherein the first subset of non-adjacentblocks are coded with a first mode and the second subset of non-adjacentblocks are coded with a second mode.

19. The method of any one or more of clauses 17-18, wherein the mergecandidate of the first type is a TMVP merge candidate.

20. The method of any one or more of clauses 17-18, wherein the mergecandidate of the first type is a last spatial merge candidate, andwherein the merge candidate of the second type is a TMVP mergecandidate.

21. The method of any one or more of clauses 17-18, wherein the firstnon-adjacent block and the second non-adjacent block are located atdifferent spatial positions with respect to the current block.

22. The method of clause 18, wherein the first mode is an advancedmotion vector prediction (AMVP) mode and the second mode is a mergemode, or vice versa.

23. The method of clause 22, wherein the merge mode is associated with amerge index which does not correspond to a spatial merge candidate.

24. The method of clause 18, wherein the first mode is a bi-predictionmode and the second mode is a uni-prediction mode, or vice versa.

25. The method of clause 18, wherein the first mode is an affine modeand the second mode is a non-affine mode, or vice versa.

26. The method of clause 18, wherein the first mode is a high motionvector (MV) precision mode and the second mode is a low MV precisionmode.

27. The method of any one or more of clauses 17-18, wherein the mergecandidate of the first type and the merge candidate of the second typeare same.

28. A method for video processing (e.g., method 3100), comprising:receiving a current block of video data; selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block;constructing at least one motion vector predictor candidate based on thefirst non-adjacent block, wherein the at least one motion vectorpredictor candidate is used as a predictor in an advanced motion vectorprediction (AMVP) mode; and decoding a block associated with the AMVPmode based on the at least one motion vector predictor candidate.

29. The method of clause 28, wherein the constructing the first motionvector predictor candidate comprises: scaling motion information of thefirst non-adjacent block to a reference picture for generating scaledmotion information; and constructing an AMVP candidate list using thescaled motion information of the first non-adjacent block.

30. The method of clause 29, wherein signaling of a reference pictureindex or a prediction direction for the current block is skipped.

31. The method of any one or more of clauses 28-30, wherein usage of themotion information from the first non-adjacent block or the secondnon-adjacent block is based on one or more of (a) an upper limit on asize of the AMVP candidate list, (b) dimensions of the current block ofvideo data, and (c) an indication of usage of the motion information.

32. An apparatus in a video system comprising a processor and anon-transitory memory with instructions thereon, wherein theinstructions upon execution by the processor, cause the processor toimplement the method in any one of clauses 1 to 31.

33. A computer program product stored on a non-transitory computerreadable media, the computer program product including program code forcarrying out the method in any one of clauses 1 to 31.

The examples described above may be incorporated in the context of themethods described below, e.g., methods 2800, 2900, 3000, and 3100 whichmay be implemented at a video decoder and/or video encoder (e.g., atranscoder).

6. Example Implementations of the Disclosed Technology

FIG. 32 is a block diagram of a video processing apparatus 3200. Theapparatus 3200 may be used to implement one or more of the methodsdescribed herein. The apparatus 3200 may be embodied in a smartphone,tablet, computer, Internet of Things (IoT) receiver, and so on. Theapparatus 3200 may include one or more processors 3202, one or morememories 3204 and video processing hardware 3206. The processor(s) 3202may be configured to implement one or more methods (including, but notlimited to, methods 2800, 2900, 3000, and 3100) described in the presentdocument. The memory (memories) 3204 may be used for storing data andcode used for implementing the methods and techniques described herein.The video processing hardware 3206 may be used to implement, in hardwarecircuitry, some techniques described in the present document.

In some embodiments, a video decoder apparatus may implement a method ofusing zero-units as described herein is used for video decoding. Thevarious features of the method may be similar to the above-describedmethods 2800, 2900, 3000, and 3100.

In some embodiments, the video decoding methods may be implemented usinga decoding apparatus that is implemented on a hardware platform asdescribed with respect to FIG. 32.

Based on the description above, one of ordinary skill in the art willappreciate that the following techniques and aspects are disclosed. Inone representative aspect, the disclosed technology may be used toprovide a method for video coding. This method includes receiving acurrent block of video data, selecting, based on a rule, a firstnon-adjacent block that is not adjacent to the current block,constructing a first merge candidate, comprising motion information,based on the first non-adjacent block, constructing a merge candidatelist according to the first merge candidate and decoding the currentblock based on the merge candidate list. If the rule is not satisfied,the motion information of the first non-adjacent block is not taken intoconsideration in the merge candidate list construction process.

In another representative aspect, the disclosed technology may be usedto provide another method for video coding. This method includesreceiving a current block of video data, selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block,constructing a first merge candidate, comprising motion information,based on the first non-adjacent block, and selecting a secondnon-adjacent block, wherein the first non-adjacent block and the secondnon-adjacent block are spatial neighbors of the current block, whereinthe first non-adjacent block is coded with a first mode, wherein thesecond non-adjacent block is coded with a second mode, and wherein thefirst non-adjacent block is selected prior to selecting the secondnon-adjacent block.

In another representative aspect, the disclosed technology may be usedto provide another method for video coding. This method includesreceiving a current block of video data, selecting, based on a firstrule, a first set of one or multiple non-adjacent blocks that are notadjacent to the current block, and constructing a first subset of one ormultiple merge candidate, comprising motion information, based on thefirst set of non-adjacent blocks with the first rule satisfied, whereina second subset of merge candidates, is constructed based on a first setof one or multiple non-adjacent blocks, satisfying a second rule,wherein the first subset of merge candidate is added into a mergecandidate list after the second subset of merge candidates.

In another representative aspect, the disclosed technology may be usedto provide another method for video coding. This method includesreceiving a current block of video data, selecting, based on a firstrule, a first set of one or multiple non-adjacent blocks that are notadjacent to the current block, and constructing a first subset of one ormultiple merge candidate, comprising motion information, based on thefirst set of non-adjacent blocks with the first rule satisfied, whereina second subset of merge candidates, is constructed based on a first setof one or multiple non-adjacent blocks, satisfying a second rule,wherein the first subset of merge candidate is added into a mergecandidate list after the second subset of merge candidates.

In another representative aspect, the disclosed technology may be usedto provide another method for video coding. This method includesreceiving a current block of video data, selecting, a first set of oneor multiple non-adjacent blocks that are not adjacent to the currentblock, and constructing a first subset of one or multiple mergecandidate, comprising motion information, based on the first set ofnon-adjacent blocks, wherein the first subset of merge candidate iscoded with the first rule satisfied, wherein a second subset of mergecandidates, is constructed based on a first set of one or multiplenon-adjacent blocks, wherein the first subset of merge candidate isadded into a merge candidate list after one merge candidate with a firsttype, and the second subset of merge candidates is added into a mergecandidate list after one merge candidate with a second type.

In another representative aspect, the disclosed technology may be usedto provide another method for video coding. This method includesreceiving a current block of video data, selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block, andconstructing a first merge candidate, comprising motion information,based on the first non-adjacent block, wherein the motion information ofthe first non-adjacent block is used as a predictor in an advancedmotion vector prediction (AMVP) mode.

From the foregoing, it will be appreciated that specific embodiments ofthe presently disclosed technology have been described herein forpurposes of illustration, but that various modifications may be madewithout deviating from the scope of the invention. Accordingly, thepresently disclosed technology is not limited except as by the appendedclaims.

Implementations of the subject matter and the functional operationsdescribed in this patent document can be implemented in various systems,digital electronic circuitry, or in computer software, firmware, orhardware, including the structures disclosed in this specification andtheir structural equivalents, or in combinations of one or more of them.Implementations of the subject matter described in this specificationcan be implemented as one or more computer program products, i.e., oneor more modules of computer program instructions encoded on a tangibleand non-transitory computer readable medium for execution by, or tocontrol the operation of, data processing apparatus. The computerreadable medium can be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter effecting a machine-readable propagated signal, or a combinationof one or more of them. The term “data processing unit” or “dataprocessing apparatus” encompasses all apparatus, devices, and machinesfor processing data, including by way of example a programmableprocessor, a computer, or multiple processors or computers. Theapparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile in a file system. A program can be stored in a portion of a filethat holds other programs or data (e.g., one or more scripts stored in amarkup language document), in a single file dedicated to the program inquestion, or in multiple coordinated files (e.g., files that store oneor more modules, sub programs, or portions of code). A computer programcan be deployed to be executed on one computer or on multiple computersthat are located at one site or distributed across multiple sites andinterconnected by a communication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Computer readable media suitable for storingcomputer program instructions and data include all forms of nonvolatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

It is intended that the specification, together with the drawings, beconsidered exemplary only, where exemplary means an example. As usedherein, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. Additionally, the use of “or” is intended to include“and/or”, unless the context clearly indicates otherwise.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document.

What is claimed is:
 1. A method for video processing, comprising:receiving a current block of video data; selecting, based on a rule, afirst non-adjacent block that is not adjacent to the current block;constructing a first merge candidate comprising motion information basedon the first non-adjacent block; identifying a second non-adjacent blockthat is not adjacent to the current block and different from the firstnon-adjacent block; based on determining that the second non-adjacentblock fails to satisfy the rule, refraining adding a second mergecandidate derived from the second non-adjacent block; constructing amerge candidate list based on the first non-adjacent block; and decodingthe current block based on the merge candidate list, wherein the ruleindicates a coding characteristic of the first non-adjacent block andthe second non-adjacent block.
 2. The method of claim 1, furthercomprising: selecting, based on the rule, a first subset of non-adjacentblocks and a second subset of non-adjacent blocks that are not adjacentto the current block; and constructing a first subset of mergecandidates comprising motion information based on the first subset ofnon-adjacent blocks and a second subset of merge candidates comprisingmotion information based on the second subset of non-adjacent blocks,wherein the first subset of merge candidates is added into a mergecandidate list after a merge candidate of a first type, and wherein thesecond subset of merge candidates is added into the merge candidate listafter a merge candidate of a second type.
 3. The method of claim 1,further comprising: constructing at least one motion vector predictorcandidate based on the first non-adjacent block, wherein the at leastone motion vector predictor candidate is used as a predictor in anadvanced motion vector prediction (AMVP) mode; and decoding a blockassociated with the AMVP mode based on the at least one motion vectorpredictor candidate.
 4. The method of claim 1, further comprising:selecting a third non-adjacent block, wherein the first non-adjacentblock and the third non-adjacent block are not adjacent to the currentblock, wherein the first non-adjacent block is coded with a first mode,and the third non-adjacent block is coded with a second mode, andwherein the first non-adjacent block is selected prior to selecting thethird non-adjacent block; and constructing a third merge candidatecomprising motion information based on the third non-adjacent block. 5.The method of claim 1, wherein the rule indicates that the firstnon-adjacent block is a merge-coded non-adjacent block, and the firstnon-adjacent block is not coded with a spatial merge candidate.
 6. Themethod of claim 5, wherein the rule indicates that the firstnon-adjacent block is coded with a motion refinement process, and thefirst non-adjacent is not coded with decoder-side motion vectorrefinement (DMVR).
 7. The method of claim 1, wherein the rule indicatesthat the first non-adjacent block is coded with a type of motioninformation that excludes motion information with low precision motionvectors or motion vector differences.
 8. The method of claim 4, furthercomprising: constructing a merge candidate list based on the first mergecandidate and the second merge candidate, wherein the second mergecandidate is added to the merge candidate list after the first mergecandidate.
 9. The method of claim 4, wherein the first mode is anadvanced motion vector prediction (AMVP) mode and the second mode is amerge mode, or vice versa, and the merge mode is associated with a mergeindex which does not correspond to a spatial merge candidate.
 10. Themethod of claim 2, wherein the first subset of non-adjacent blocks arecoded with a first mode and the second subset of non-adjacent blocks arecoded with a second mode.
 11. The method of claim 10, wherein the mergecandidate of the first type is a TMVP merge candidate.
 12. The method ofclaim 10, wherein the merge candidate of the first type is a lastspatial merge candidate, and wherein the merge candidate of the secondtype is a TMVP merge candidate.
 13. The method of claim 10, wherein thefirst non-adjacent block and the second non-adjacent block are locatedat different spatial positions with respect to the current block. 14.The method of claim 10, wherein the first mode is an advanced motionvector prediction (AMVP) mode and the second mode is a merge mode, orvice versa.
 15. The method of claim 14, wherein the merge mode isassociated with a merge index which does not correspond to a spatialmerge candidate.
 16. The method of claim 3, wherein the constructing thefirst motion vector predictor candidate comprises: scaling motioninformation of the first non-adjacent block to a reference picture forgenerating scaled motion information; and constructing an AMVP candidatelist using the scaled motion information of the first non-adjacentblock.
 17. The method of claim 16, wherein signaling of a referencepicture index or a prediction direction for the current block isskipped.
 18. The method of claim 16, wherein usage of the motioninformation from the first non-adjacent block or the second non-adjacentblock is based on one or more of (a) an upper limit on a size of theAMVP candidate list, (b) dimensions of the current block of video data,and (c) an indication of usage of the motion information.
 19. Anapparatus in a video system comprising a processor and a non-transitorymemory with instructions thereon, wherein the instructions uponexecution by the processor, cause the processor to: receive a currentblock of video data; select, based on a rule, a first non-adjacent blockthat is not adjacent to the current block; construct a first mergecandidate comprising motion information based on the first non-adjacentblock; identify a second non-adjacent block that is not adjacent to thecurrent block and different from the first non-adjacent block; based ondetermining that the second non-adjacent block fails to satisfy therule, refrain from adding a second merge candidate derived from thesecond non-adjacent block; construct a merge candidate list based on thefirst non-adjacent block; and decode the current block based on themerge candidate list, wherein the rule indicates a coding characteristicof the first non-adjacent block and the second non-adjacent block.
 20. Anon-transitory computer readable storage medium storing instructionsthat cause a processor to: receive a current block of video data;select, based on a rule, a first non-adjacent block that is not adjacentto the current block; construct a first merge candidate comprisingmotion information based on the first non-adjacent block; identify asecond non-adjacent block that is not adjacent to the current block anddifferent from the first non-adjacent block; based on determining thatthe second non-adjacent block fails to satisfy the rule, refrain fromadding a second merge candidate derived from the second non-adjacentblock; construct a merge candidate list based on the first non-adjacentblock; and decode the current block based on the merge candidate list,wherein the rule indicates a coding characteristic of the firstnon-adjacent block and the second non-adjacent block.